BUGFIX: default amount should be 1

This seems to have been broken for a little while.

The bug was that if you just left the donation combo-box as is (i.e. at
‘1.00’), entered your card details and clicked ‘donate’ you would get
an error, because the default ``amount`` property was null rather than
0.

assets/javascripts/discourse/components/stripe-card.js.es6

@@ -11,7 +11,7 @@ export default Ember.Component.extend({
     { value: 50, name: '50.00'}
   ],
   result: [],
-  amount: null,
+  amount: 1,
   stripe: null,
   transactionInProgress: null,
   settings: null,